Described herein are methods of writing and reading flash memories, as well as flash memory devices and systems that are so read and written.
Flash memories such as NAND flash memories are structured as rectangular arrays of memory cells. The cells are arranged in orthogonal bit lines and word lines. One or more bits of data are written to each cell by injecting sufficient electrical charge into a floating gate of the cell to place the cell's threshold voltage within a range of threshold voltages that represents the value of that bit or of those bits. A flash memory cell is read by comparing its threshold voltage to reference voltages that mark the boundaries between threshold voltage ranges. In the case of a NAND flash memory, the cells are written and read one full word line at a time. The word lines are grouped further into blocks, such that cells are erased an entire block at a time.
The word lines of a given block are written sequentially. After the first word line of a block is written, writing the subsequent word lines of the block may disturb the previously written word lines, because injecting electrical charge into the floating gates of the cells of one word line may also affect electrical fields in the cells of neighboring word lines, thereby increasing the threshold voltages of the cells of a nearby, previously written word line, possibly to the point of changing the bit values that are represented by the threshold voltages of the previously written nearby word line. This coupling between successively written flash memory cells is called the Yupin effect.
Two general methods are known for mitigating the Yupin effect. The first method is: whenever reading a word line other than the last written word line of a block, first read also one or more word lines of the block that were written after the target word line was written, adjust the reference voltages that are to be used to read the target word line according to the threshold voltages of the cells of the other word lines, as read, and use the adjusted reference voltages to read the target word line. The disadvantage of this method is that it slows down the reading of the target word line by requiring that two or more word lines be read in order to correctly read the target word line. The second method is to write the word lines of a block, except for the last word line, by injecting somewhat less than enough charge into the floating gates of the cells to raise the cells' threshold voltages to the desired level. Then, for each target word line, after the other word lines that could disturb the writing of the target word line have been so written, more electrical charge is injected into the cells of the target word line whose threshold voltages have not been raised to their desired values by the disturbances associated with the writing of the other word lines, to raise these threshold voltages to their desired values. The disadvantage of this method is that it slows down the writing of the word lines. Essentially, all but the last word line of the block must be written twice.